1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication method, and a wireless access point device.
Priority is claimed on Japanese Patent Applications No. 2012-041263, filed Feb. 28, 2012, and No. 2012-255203, filed Nov. 21, 2012 the contents of which are incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
Conventionally, distributed control systems (DCS) are constructed in industrial plants and factories and the like. In these DCS, on-site instruments (such as measuring instruments and operating machinery) known as field devices that are used to perform high-level automated operations, and a control unit that controls these instruments are connected together via a communication device. Most of the communication systems that form the basis of this type of distributed control system have been communication systems that communicate by wires, however, in recent years, systems that perform wireless communication based on industrial wireless communication standards such as ISA 100.11a and Wireless HART (Registered Trademark) have been developed.
Communication systems that are based on the aforementioned industrial wireless communication standards are formed, broadly speaking, by field devices that are able to communicate wirelessly (i.e., wireless field devices), a wireless access point device, and a control device. A wireless access point device is a device that is connected to a control unit via a communication line, and is installed in a location that allows it to communicate wirelessly with wireless field devices, and that relays various types of data that are exchanged between the wireless field devices and the control unit. A control unit is a device that controls the wireless field devices by transmitting and receiving various types of data (for example, measurement data and control data) with the wireless field devices via the wireless access point device.
In a communication system that is based on the aforementioned ISA 100.11a, the data transfer speed between the wireless field devices and the wireless access point device is set at, for example, 250 kbps (bit per second). Because the wireless access point device normally performs wireless communication with a plurality of wireless field devices, the data transfer speed between the wireless access point device and the control unit must be set to a faster speed than the data transfer speed (250 kbps) between the wireless field devices and the wireless access point device. Because of this, the wireless access point device and the control unit are connected together via a high-speed communication line such as, for example, an Ethernet (Registered Trademark) cable.
An example of a wireless communication system in which wireless communication that is based on the aforementioned ISA 100.11a is possible is disclosed in “Field Wireless Solution Based on ISA 100.11a to Innovate Instrumentation” in the Yokogawa Technical Report Vol. 53, no. 2, 2010 by Shuji Yamamoto, et al. A system in which the transmitting and receiving of data, as well as the supplying of power are achieved via an Ethernet (Registered Trademark) cable is disclosed in Japanese Unexamined Patent Application, First Publication No. 2005-244830. An example of a power line communications (PLC) system in which the transmitting and receiving of data is possible via a power line is disclosed in Japanese Unexamined Patent Application, First Publication No. 2006-148593.
Generally, while it is common for wireless field devices to be installed in hazardous locations such as areas where inflammable gas is used, the control unit is normally installed away from the hazardous location in a non-hazardous location (i.e., in a safe area where inflammable gas or the like is not being used). Because it is necessary for the above-described wireless access point device to maintain wireless transmission quality between itself and the wireless field devices, the wireless access point device cannot be installed in a location further than several hundred meters from the wireless field devices, and often the wireless access point device ends up being installed in a location having the same hazard level as the wireless field device.
Here, the maximum length of an Ethernet (Registered Trademark) cable is prescribed by a Standard. For example, in the 10 Base-T and 100 Base-T Standards, the maximum length of the cable is prescribed as 100 meters. Because of this, if the distance between a hazardous location and a control unit that is installed in a non-hazardous location is approximately several hundred meters, then it is quite possible that the wireless access point device that is connected to the Ethernet (Registered Trademark) cable will be located in the hazardous location. However, if the distance between the hazardous location and the control unit that is installed in a non-hazardous location is approximately several kilometers, then it is not possible for the wireless access point device that is connected to the Ethernet (Registered Trademark) cable to be installed in the hazardous location, and the distance between the wireless field devices and the wireless access point device ends up extending to several kilometers, and the problem arises that it becomes impossible for communication to be performed between the wireless field devices and the control unit.
Here, although it does become possible to extend the wire length between the wireless access point device and the control unit if a repeater is installed, the problem arises that this need to install a repeater causes the costs the costs to increase by a corresponding amount. This is also the case when the Ethernet (Registered Trademark) cable disclosed in Japanese Unexamined Patent Application, First Publication No. 2005-244830 which is capable of both transmitting and receiving data as well as supplying power is used.
Moreover, in an Ethernet (Registered Trademark) that employs an optical fiber, it is possible to extend the wire length between the wireless access point device and the control unit to a range of several kilometers without having to use a repeater. However, in an Ethernet (Registered Trademark) that employs an optical fiber, it is necessary to provide an optical converter to convert electrical signals into optical signals, and wiring to supply power to the optical converter. As a consequence, the problems arise that not only does the power consumption increase, but the costs also increase.